1. Field of the Invention
The invention is directed to a washer comprising a central opening for receiving a screw, particularly for fastening single-layer or multiple-layer roofing, including an insulating layer, on a fixed underlying structure.
2. Description of the Prior Art
An apparatus for fastening roofing on soft insulating material at a fixed supporting base in which washers having a large surface area are used, the washers being pressed on with a self-boring and automatic thread-cutting screw, is already known (AT-PS 380 051). In such a washer, a central recess is provided in a flat main part for receiving the screw head, wherein an opening for a screw is arranged at the base of this recess. A washer can only be constructed in this way with relatively soft flexible material which must be fastened to a fixed underlying structure. That is, it must be possible to press the central recess of the disk into the part to be fastened.
Fastening arrangements are also known in which a washer consisting of flexible plastic is used (CH-PS 612 718 or DE-OS 35 00 084). In such arrangements, the edge areas, which project outwardly relatively far, are too weak in constructions having a correspondingly large surface area to exert a corresponding drive force, or it is necessary to arrange ribs on the underside or the upper side of the washers, which ribs project radially outwardly from the central opening. However, this again involves substantial problems for magazine loading of the washers. It is essential, especially in series fastening on a roof, that both the screws and the washers be fed to the screwing apparatus in a magazine-loaded form and that they can be readily separated from the magazine and each other. However, such a possibility is excluded if tubular parts which project relatively far in a downward direction are provided at the washer itself for receiving and holding the screw shaft.
In another known construction (DE-GM 78 12 641), a flat washer is provided in which annularly extending flanges are provided coaxially relative to the central through-opening for receiving the screw. Such a construction of a washer is also not suitable for magazine loading and automatic separation in an automatic screw machine, since constant interference would occur because of the transversely extending ribs. The same is also true for washers (FR-PS 1 362 380) in which tips are provided which project downwardly from the flat washers in the direction of the material to be fastened. Also, in such a construction, only a manual single-feed, and accordingly only a manual use of the fastener, is possible.
In another known arrangement (DE-OS 31 37 836), a plastic washer is provided which comprises a cylindrical part for receiving and guiding a screw, which cylindrical part projects forwardly relatively far at the underside of the washer. Also, in this arrangement, ribs, which prevent separation in an automatic screw machine, must be provided at the underside of the washer for achieving the necessary stability. These requirements also apply to a construction according to DE-OS 31 06 844.
Washers on which relatively long, tubular bodies are provided for the displaceable holding of a screw shaft and a screw head are not suitable for preloading in magazine form (GB-PS 1 499 224). In such washers, an orderly load-distributing contact pressing force could only be achieved if the washer itself were equipped with ribs.